Introduction of Direct Memory Access (DMA)

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Introduction of Direct Memory Access (DMA)
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Why is DMA?

• It is wasteful to feed data into a controller
  register 1 bytes at a time. (PIO)
• The DMA unit is word.
• In the high loading environment, a system with
  DMA has better improvement.

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DMA transfer
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DMA Progress

• To initiate a DMA transfer, the host writes a DMA
  command into memory
• A pointer to the source of a transfer
• A count of the number of bytes to be transferred
• ..
• The CPU writes the address of the DMA command
  block to the DMA controller.

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DMA Progress (cont.)

• The DMA controller proceeds to operate the memory
  bus directly without CPU help.
• Handshaking exists between DMA controller and
  device controller.
• When the entire transfer is finished, the DMA
  controller will interrupts the CPU.

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Handshaking

• DMA-request and DMA_acknowledge
• When a word of data is available, the device
  controller places a signal on the DMA-request
  wire.
• The signal causes the DMA controller to seize the
  memory bus,
• To place the desired address on the
  memory-address wire
• To place a signal on the DMA-acknowledge wire

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Handshaking (cont.)

• When the device controller receives the
  DMA-acknowledge signal,
• it transfer the word of data to memory
• and remove the DMA_request signal.